The present invention relates to a method of traction control to stabilize the driving of a motor vehicle in the case of increased slip at the driving wheels, and more particularly, to a method which uses braking interventions at the driving wheels in the event of increased slip at driving wheels as a function of whether the motor vehicle is travelling around a curve or in a straight line and as a function of the vehicle speed.
It is known that the spinning of the driving wheels, that is to say the appearance of an undesirably high driving wheel slip which reduces traction, can be prevented by braking interventions of an acceleration skid control system (ASR). One driving wheel, specifically that with the lower adhesion (the so-called low-adhesion wheel) frequently begins to spin first, particularly in the case of different coefficients of friction, with the result that there is a loss of driving torque. In order to combat this driving torque loss, an electronic traction system can be provided which undertakes a one-sided braking intervention at the low-adhesion wheel. This active braking on the low-adhesion wheel not only brings it back into a generally more favorable slip range, but also acts simultaneously as a limited-slip torque to increase the traction at the opposite driving wheel, i.e. the wheel with the greater adhesion (the so-called high-adhesion wheel). When the adhesion is no longer sufficient, the wheel slip at the high-adhesion wheel can also become excessive so that the driving stability can be impaired by such braking interventions to increase traction, particularly in the case of high vehicle speeds and on curves.
The article "Ausgebremst" (translation, "Fully Braked") in the "Auto-Motor-Sport" Journal, p. 34 (2.8.1986), describes an electronic differential lock, in which the effect of increasing the traction at the high-adhesion wheel, caused by the effects of braking at the low-adhesion wheel, is used to increase traction.
The difficulty of adequate driving stability, such as is also known from ASR systems, occurs in such electronic traction systems, and for this reason the known electronic differential lock mentioned above, is used only as a pull-away aid and is automatically switched off at vehicle speeds above 40 km/h.
DE 31 27 301 A1 describes a method of traction control to stabilize driving which operates on the so-called select-low principle. That is, an intervention controlling traction takes place on condition that the low-adhesion wheel slip is above a specified limit. As a further measure to stabilize driving, such a traction-control intervention takes place as soon as the system recognizes that the vehicle is travelling round a curve at a speed already in excess of a lower limiting speed of, for example, 40 km/h whereas, when the vehicle is travelling in a straight line, such an intervention is only undertaken when a higher limiting speed of, for example, 100 km/h is exceeded. In the case of vehicle speeds below the lower limiting speed, no traction control intervention to reduce the driving torque and stabilize driving takes place. The traction control interventions provided include subjecting the low-adhesion wheel to brake pressure, with an attempt being made to set a brake pressure which leads to maximum traction torque at the high-adhesion wheel. If the brake disc temperature exceeds a specified threshold value due to these braking interventions, the traction control device is switched off for a specified time interval.
DE 35 18 221 C2 describes a brake system in which measures are taken to stabilize driving during braking procedures when travelling around curves. These measures include the determination of a yaw angle reference value as a function of the vehicle transverse forces, of the vehicle speed and of the distance travelled around the curve as well as the activation of the front and rear wheel brakes, when braking is demanded during travelling around a curve, so that the actual yaw angle value derived from the vehicle transverse forces recorded is adjusted to the yaw angle reference value. This takes place automatically by way of an electronic control unit.
DT 2 319 862 describes an anti-lock system in which, to increase driving stability, two electronic analysis circuits of the wheels of one axle are configured in such a way and are associated with one another such that the brake pressure of one wheel is retained, built up or lowered in order to avoid large brake force differences at the wheels of one axle and/or to achieve a common control cycle variation from an output signal of the other wheel or, during the individual occurrence of the output signal, on the wheel which has to be influenced. In the case of a pair of signals which are time displaced relative to one another, the leading or trailing signal can be used for individual occurrence of a signal can be used for control.
It is an object of the present invention to provide a traction control method to stabilize driving so that a high level of driving stability can be maintained in continuous driving operation even in the case of vehicles in which braking interventions to regulate acceleration skid, and in particular one-sided braking interventions to increase traction, are undertaken at the driving wheels.
This object has been achieved in accordance with the present invention by a method involving monitoring whether high-adhesion wheel slip is above a specified limit and whether vehicle speed is in a range between a specified lower limiting speed and a specified upper limiting speed and, if the response is positive, using the high-adhesion wheel slip as the control parameter; effecting brake pressure at the driving wheels on both sides of the motor vehicle with synchronous brake pressure control upon recognizing that the motor vehicle is travelling around a curve, and reducing brake pressure which increases traction at the low-adhesion wheel upon recognizing that the motor vehicle is travelling in a straight line.
In the event of excessive high-adhesion wheel slip, the method reacts in a different manner to stabilize driving, depending on whether the vehicle is travelling around a curve or in a straight line.
When the vehicle is travelling around a curve, the brake pressure is controlled synchronously on both sides to stabilize the driving on the curve, with the high-adhesion wheel slip being the control parameter which is controlled within a range which ensures sufficient cornering force for the vehicle. The cornering force at the high-adhesion wheel can be maintained by control based on the high-adhesion wheel slip and not on the low-adhesion wheel slip. The low-adhesion wheel is spinning and no longer possesses any cornering force.
Driving instabilities can be prevented by also specifically increasing the brake pressure at the high-adhesion wheel. These driving instabilities are particularly due to the yaw velocity present when the vehicle is travelling around a curve where one-sided braking intervention controlling traction takes place on a condition that the low-adhesion wheel slip is above a low-adhesion wheel when the vehicle is travelling in a straight line, the low-adhesion wheel can be prevented from accelerating by the driver reducing the driving torque, should this be desired.
The present invention has the advantage that the braking intervention which places a load on the brakes on both sides during travel around a curve, remains activated for at most a specified period, with the result that excessive brake heating is prevented. In order to avoid disturbing control oscillations, furthermore, such a braking intervention is deactivated whenever the amount of curvature, or the high-adhesion wheel slip, falls below respectively specified deactivation limits, which are preferably smaller than the activation limits which, when exceeded, activates the braking intervention.
A further feature of the present invention is that brake pressure at the low-adhesion wheel to increase traction is retained when the vehicle speed is below a specified minimum speed even if the vehicle is travelling in a straight line and there is increased high-adhesion wheel slip. This brake pressure retention permits a certain increased high-adhesion wheel slip in the low speed range in order to achieve maximum traction by way of a scraping effect, the driving stability not being noticeably impaired in this range of low speeds.
It has also been found beneficial for the speed range with active stability control to be from 15 km/h to 80 km/h. In this speed range, travel around a curve can be recorded very reliably by wheel rotational speed sensor techniques alone without a transverse acceleration sensor, steering angle sensor or yaw velocity sensor. Below 15 km/h, the driver can still react sufficiently rapidly, it being the case, in addition, that fairly large skidding movements, for example escape turns, remain possible.